1. Field of the Invention
This present invention relates to a write head for an optical printer, and more particularly to a write head for an optical printer which comprises a vacuum fluorescent tube using the principle of a fluorescent display tube.
2. Description of the Prior Art
An optical printer, as illustrated in FIG. 1, typically includes a record medium 101 such as a photosensitive drum, a belt or the like, an electrifier 102 for uniformly electrifying a surface of the record medium 101, a write head 103 for irradiating light on the electrified record medium while carrying out an ON-OFF operation in dependence upon a write signal, a developing device 104 for adhering toner to a portion of the record medium from which charge is removed due to the irradiation during the exposure step, a transfer device 105 for carrying out the transfer on to decalcomania paper after the development step, a fixing device 106 for carrying out the fixing on the transferred decalcomania paper by heat or the like, an erasing device 107 for removing charge from the surface of the record medium 101 after the transfer step, and a cleaning device 108 for removing toner remaining on the surface of the record medium 101 and cleaning the surface.
The optical printer is classified depending upon the type of write head used in the exposure step.
One type is the laser printer which uses a laser beam emitting element as its write head and is adapted to scan the record medium while carrying out ON-OFF switching of the laser beam. Unfortunately, this has the disadvantage of being complicated and large-sized. Also, the laser printer takes a considerable length of time to reach high-speed stability since it requires a drive mechanism for a high speed rotation mirror to carry out the scanning of the laser beam. It has a further disadvantage in that its reliability deteriorates as the rotating portion of the drive mechanism etc. wears.
Another optical printer is the LED printer which uses an LED (light-emitting diode) as its write head. However, techniques for connecting a large number of LED elements to one another are highly complicated and troublesome. Also, the LEDs are elements independent from one another, so that dispersion in luminance occurs between the LEDs. Accordingly, it requires a difficult and complicated LED selecting operation to obtain uniform luminance.
In order to eliminate the problems described above, a vacuum fluorescent tube using the principle of a fluorescent display tube was proposed as a write head for an optical printer.
For example, Japanese Patent Application Laid-Open Publication No. 46740/1984 discloses a vacuum fluorescent tube for an optical printer which uses a static driving system of 1 in duty and a dynamic driving system of 1/2 in duty. However, a vacuum fluorescent tube for an optical printer requires a large number of emission dots, for example, as many as above 3000 per line of A4 size paper, resulting in the need for very large numbers of anode leads and driving integrated circuits which in turn result in a substantial increase in manufacturing costs.
In view of the foregoing, the present inventors proposed the use of, a vacuum fluorescent tube as a write head for an optical printer, such as the dynamic driving system shown in FIG. 2.
This proposed write head includes a substrate 1, a plurality of strip-like anode conductors 2 arranged in parallel at intervals on the substrate 1, and a plurality of strip-like grids 3 arranged in parallel at intervals above the anode conductors 2 so as to extend obliquely across the anode conductors. The grids 3 each are formed like a flat plate and each have a slit 4 at their central portion which extends in the oblique direction across the anode conductors 2. The anode conductors 2 each have a phosphor layer 5 deposited at intervals opposite the slits 4 of the grids 3 and adjacent thereto, so that an anode 6 may be formed. Each of the phosphor layers 5 obliquely arranged along the slits 4 constitutes a group of picture cells.
The write head also includes filaments stretched above the grids 3. Reference numerals 8 and 9 designate a front cover and side plates, respectively, which form a sealed casing together with the substrate. The casing is kept in an evacuated state.
Electrodes, such as grid terminals, lead out from the sealed casing and the like and are connected to a driver circuit (not shown), so that the anodes 6 each may be scanned by means of a time-division pulse signal and a positive display pulse may be applied to the grids 3 desired in synchronism with the scanning, resulting in any dot-like fluorescent phosphor layers 5 selectively carrying out emission. However, electrons emitted from the filaments 7 often impinge those phosphor layers 5 from which emission is not desired, so causing leakage emission. In order to prevent such a defect, the vacuum fluorescent tube is constructed to apply a negative voltage of the grids 3 for the phosphor layers 5 which are not desired to carry out emission.
The vacuum fluorescent tube constructed described above is arranged in the optical printer so that the axis of the photosensitive drum acting as the record medium is parallel to the direction of arrangement of the anode conductors 2. This causes the arrays of the dot-like phosphor layers 5 belonging to each group to be oblique with respect to the axis of the photosensitive drum. However, a suitable electrical signal treatment required to adjust suitably the timing of emission of each phosphor layer 5 depending upon the rotational speed of the photosensitive drum results in light irradiated from each of the phosphor layers 5 forming a straight line on a surface of the photosensitive drum parallel to its axis. Thus, the vacuum fluorescent tube allows for an interval between adjacent phosphor layers which is larger than the dot pitch in printing, resulting in many advantages in manufacture, because light emitted from each of the obliquely arranged phosphor layers 5 of each group is irradiated to a position in a line on the surface of the photosensitive drum.
As described above, in the vacuum fluorescent tube for a write head for an optical printer, a negative voltage (cut-off bias) is applied to the grids 3 corresponding to the phosphor layers 5 which are not desired to carry out emission, for the purpose of preventing leakage emission. However, this is disadvantageous in that electrons are subjected to repulsion due to the negative voltage generated by the cut-off bias, so that the direction of travel of the electrons may be deflected. This causes the electrons to fail to impinge on the phosphor layers which are intended to carry out emission, resulting in eclipse or shading which is the phenomenon occurring when a part of a phosphor layer fails in emission.
FIG. 3 shows diagrammatically the positions of the electrodes in a conventional vacuum fluorescent tube in which a cut-off bias is applied to a part of the grids 3a and equipotential lines produced by a voltage applied thereto. Electrons emitted from the filaments describe loci perpendicular to the equipotential lines. Accordingly, in the vicinity of the grids 3c, the electrons are downwardly accelerated due to a positive potential of the grids 3c, as diagrammatically shown in FIG. 4, resulting in impingement on the phosphor layers 5; whereas, in the vicinity of the grids 3b, the direction of travel of the electrons is substantially deflected due to a negative voltage of the adjacent grids 3a, so that the electrons are caused to impinge on only a part of the phosphor layers 5 which are desired to carry out emission, resulting in eclipse.
Also, the application of a cut-off bias to both grids 3 adjacent to the grids 3 to which a positive voltage is applied causes a decrease in luminance due to a decrease in the number of electrons impinging on the phosphor layers, although this often prevents the occurrence of eclipse.
Furthermore, the conventional vacuum fluorescent tube has another disadvantage in that it is necessary to align vertically the position of the slit 4 of each grid 3 precisely with that of the corresponding phosphor layer 5 deposited in a dot-like manner on the anode conductor 2, resulting in difficulties in manufacture.